To provide the surface of substrates composed of plastic films with specific functions, as surface processing methods which laminate various kinds of thin films, employed have been a vacuum deposition method, a sputtering method, an ion beam method, and an ion plating method, as well as a plasma chemical vapor deposition (CVD) method utilizing glow discharge under reduced pressure. However, any of these methods are composed of vacuum based processing means, and pressure of processing systems is required to be significantly reduced. Consequently, an employed film forming apparatus becomes large scale instruments and units such as large processing chambers or large vacuum pumps which require complicated operations under high vacuum. Further, these instruments and units result in various types of restrictions and limitations such as sizes such as diameter and width of the wound roll of a substrate, capacity of thin film forming materials, or the like.
In order to overcome drawbacks of the surface processing methods composed of the above vacuum based processing means, thin film formation, via a normal pressure plasma CVD method, has been tried. In an apparatus which is employed for continuous surface processing or continuous film formation employing the above normal pressure plasma, in order to minimize point discharge, employed is a pair of parallel plate electrodes rimming the periphery with a smooth curved surface.
The above parallel plate electrodes are easily prepared, while the distance between them is easily arranged. Further, a large electrode area is usable. Consequently, a film to be processed, which is conveyed between the electrodes, is sequentially processed, whereby it is possible to enhance the film forming rate, and in addition, compared to the above-mentioned low pressure plasma, it is possible to increase the density of the plasma processing gasses, resulting in a method of excellent processing efficiency. However, from another aspect, installation cost such as electrodes is high and reduction of equipment cost or cost reduction via an increase in processing capacity are keys for practical application. In order to increase the above processing capacity, it is possible to consider an increase in energy such as an increase in plasma density or an increase in electric field intensity. However, when the electric field intensity is increased, concentrated discharge of large electric current may occur due to arc.
Further, since the above electrodes are fixed, they are always exposed to a flow of a gas mixture for film formation and plasma discharge is continued, the surface of the above electrodes is gradually stained, and finally, the discharge state is adversely affected to result in fluctuation of performance of the formed film and processing surface. In a marked case, problems occur in which clearly visible defects such as streaking or mottling are generated.
In order to minimize staining of the above electrodes, a discharge plasma processing system is proposed in which a gas mixture is fed between the plasma processing electrodes, each of which is in the shape of a roller (refer, for example, to Patent Document 1). However, it is difficult to uniformly introduce a processing gas between the roller electrodes which face each other. Namely, due to gas leakage (counter flow) from the gap between the gas feeding nozzle and the roller electrode, and direct adhesion of retained components on the substrate prior to conveyance to a discharge space between the feeding exit and the discharge space, targeted film quality has not been realized.
As means to minimize the gas leakage, for example, Patent Document 2 describes installation of a skirt which minimizes leakage of introduced gasses. However, no technical disclosure is made to overcome drawbacks such as formation of retained components between the feeding exit and the discharge space, and direct adhesion of raw materials onto a substrate prior to introduction to the discharge space.
Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2003-93870
Patent Document 2: JP-A No. 2001-279457